The present invention relates to a system apparatus for removing nitrogen oxides (NOx) from combustion turbine exhaust gases and more particularly to apparatus for removing NOx from combustion turbine exhaust gas at exhaust gas temperature conditions existing in a combined cycle power plant with a waste heat recovery arrangement preceding the steam cycle portion.
In the operation of combustion turbines, NOx originates from nitrogenous compounds in the fuel used and from atmospheric nitrogen fixation during combustion. Natural gas is a relatively clean fuel from the standpoint of fuel bound NOx, but coal and oil can vary significantly in NOx content.
It is necessary for environment control that the plant be operated so that the combustion process is conducted without excessive NOx emissions and/or that the combustion exhaust gas be processed to remove NOx to less than allowable limits. It is desirable that NOx emission control be provided with little or no cost in plant operating efficiency.
Although some turbine combustion processes may be conducted with NOx emissions within allowed limits, there is a need for post-combustion NOx removal in many current design combustion turbines as well as many past turbine installations. Where the flame temperature must be reduced to the point where plant efficiency is unacceptable in order to bring the combustion process within NOx emission limits, there is in fact a requirement for a NOx removal system.
One prior art arrangement for dry catalytic removal of NOx from boiler flue gases in a conventional steam power plant involves the injection of a reductant such as ammonia (NH.sub.3) or carbon monoxide (CO) etc. into the exhaust gas. After catalyzed chemical reaction, NOx is converted into nitrogen (N.sub.2) and water (H.sub.2 O).
For the NOx removal process to be effective in the dry catalytic removal system, the exhaust temperature typically must be within a determinable temperature range, i.e. within 610.degree. F. to 810.degree. F. in one presently available NOx removal system which uses NH.sub.3 as a reductant. Above 810.degree. F., NH.sub.3 detrimentally begins to cause additional NOx generation and below 610.degree. F. it detrimentally scavenges for SOx to form ammonium sulfate and ammonia bisulfate.
Another known kind of NOx removal process involves injection of NH.sub.3 and heavy water (H.sub.2 O.sub.2) into the exhaust gas from a combustion turbine. This wet process is described more fully in an ASME publication entitled NOx Removal Process by Injection of NH.sub.3 and H.sub.2 O.sub.2 in Gas Turbine Exhaust Gas and presented by Hitachi Ltd. personnel at the San Diego Gas Turbine Conference on Mar. 12-15, 1979.
Some background description on the dry type NOx removal process is set forth in (1) a publication entitled Development of NOx Removal Processes With Catalyst for Stationary Combustion Facilities and published by Mitsubishi Heavy Industry in a May 1977 Mitsubishi Technical Bulletin Number 124 and (2) another publication entitled Hitachi Zosen DeNOx Process For Fossil Fuel Fired Boilers presented by personnel of Hitachi and Chemico Air Pollution Control Corporation on Nov. 1978 at Westinghouse, Eddystone, PA. In the latter publication, some consideration is given to temperature control of boiler exhaust gas in the conventional steam plant application of the dry NOx removal process.
In the known proposals for combustion turbine application of the dry NOx catalytic removal systems, a NOx removal module is installed so that exhaust gas is mixed with cooler ambient air if it is too hot or with hot bypassed combustion gas if it is too cool to produce a gas mix at optimum temperature or at least within the required temperature range.
While NOx is removed from exhaust gas in the known related prior art, energy is consumed in providing for the NOx removal, as by gas mixing to regulate the exhaust gas temperature within range in the dry catalytic system. Energy consumption for NOx removal can reduce plant efficiency significantly.
In the cited copending application, a new system is disclosed for implementing a NOx removal process in a combustion turbine plant. The present application is directed to the implementation of a NOx removal process in the specific case of a combined cycle power plant which includes at least one combustion turbine. It is desirable that NOx removal in a combined cycle plant be both effective and efficient.